Furchgott (Nature, 1980, 288:373-6) reported in 1980 that endothelial cells release a powerful vasodilator which is termed endothelium-derived relaxing factor (EDRF). Subsequent research has shown that many endothelium-dependent receptor agonists, including, for example, adenosine diphosphate (ADP), adenosine triphosphate (ATP), 5-hydroxytryptamine (5-HT), thrombin, acetylcholine (ACh), vasoactive intestinal polypeptide (VIP), oxytocin, cholecystokinin (CCK), calcitonin gene-related peptide, noradrenaline, histamine, calcium ionophores, melittin and ergometrine invoke the release of EDRF. The release of EDRF, in turn, stimulates the soluble form of the enzyme guanylate cyclase, thereby increasing levels of the second messenger, cyclic guanosine monophosphate (cGMP), which, in turn, produces vasorelaxation. Reviews are available which discuss this process in more detail (see, for example, A. M. Katz, J. Am. Coll. Cardiol., 1988, 12: 797-806; J. A. Angus and T. M. Cocks, Pharmaceutical Therapeutics, 1989, 41: 303-52; S. A. Waldman and F. Murad, Pharmacological Reviews, 1987, 39: 163-196; F. Murad, J. Clin. Invest., 1986, 78: 1-5; L. J. Ignarro, Biochem. Pharmacol., 1991, 41: 485-90; and S. Moncada, R. M. J. Palmer and E. A. Higgs, Pharmacological Reviews, 1991, 43: 109-142).
Pharmacological characterization of EDRF and its effects has been an active area of research over the past eleven years (K. Shikano et al., J. Pharmacol. Exp. Therap., 1988, 247:873-81 and L. J. Ignarro, Annu. Rev. Pharmacol. Toxicol., 1990, 30: 535-60), and now there is substantial evidence that nitric oxide (NO) is the major endothelium-derived relaxing factor (R. M. J. Palmer et al., Nature, 1987, 327: 524-6; S. Moncada et al, Biochem. Pharmacol., 1989, 38: 1709-15; and S. Moncada et al., Hypertension, 1989, 12: 365-72). In particular, nitric oxide (NO) was tested and found to elicit a potent and transient relaxation of bovine coronary artery accompanied by cGMP accumulation (C. A. Guetter et al., J. Cyclic Nucleotide Res., 1979, 5:211-24) and it was also shown to activate soluble guanylate cyclase and to elevate tissue cGMP levels.
Recent reports (H. H. H. W. Schmidt et al., European J. Pharmacol., 1988, 154:213-6 and S. Moncada et al., Hypertension, 1988, 12: 365-72) have suggested that L-arginine may be the endogenous source of EDRF (NO), and this hypothesis is further supported by the observation that EDRF (NO) production is inhibited by the simple arginine derivative, N.sup.G -methylarginine (R. M. J. Palmer et al., Biochem. Biophys. Res. Comm., 1988, 153: 1251-56; S. Moncada et al., Biochemical Pharmacology, 1988, 37: 2495-2501; and I. Sakuma et al., Proc. Nat. Acad. Sci. USA, 1988, 85: 8664-7).
Increasing evidence has been uncovered that suggests EDRF or EDRF-like substances may also control cGMP production in non-endothelial cells (J. Garthwaite, Nature, 1988, 336:385-388 and T. J. Rimele et al., J. Pharmacol. Exp. Therap., 1988, 245:102-111) and that this method of guanylate cyclase regulation may be ubiquitous. A role in the regulation of neural transmission and a role in the neural control of gastrointestinal smooth muscle function has been elucidated (J. Collier and P. Vallance, Trends in Pharmacological Sciences, 1989, 428-31 and K. M. Desai et al., Nature, 1991, 351: 477-9). Compounds that control, inhibit, or otherwise regulate this pathway, therefore, have potentially many and varied therapeutic applications, for instance, as analgesics (Duarte et al., European J. Pharmacology, 1990, 186: 289-93), as cerebroprotectives (cf. Southham et al., J. Neurochem., 1991, 56: 2072-81) and as hypocholesteremics (Cooke et al., Circulation, 1991, 83: 1057-62).
Recent work has shown that there are many isoforms of the EDRF (NO) synthase enzyme. The primary distinction between these isoforms is whether they are constitutive or inducible forms, but other factors which serve to distinguish these isoforms are their cellular localization and their cofactor requirements. Many of these isoforms have been arbitrarily given Roman numeral designations and are described in the table below, wherein NADPH represents reduced nicotinamide adenine dinucleotide phosphate, BH.sub.4 represents tetrahydrobiopterin, FAD represents flavin adenine dinucleotide and FMN represents flavin mononucleotide.
__________________________________________________________________________ Cosubstrates % Regulated M.sub.r of denatured Present Type Cofactors by protein* in __________________________________________________________________________ I NADPH, Ca.sup.++, 155 kDa** brain and (soluble) BH.sub.4 calmodulin cerebellum II NADPH, BH.sub.4, induced by 125-135 kKa** macrophages (soluble) FAD/FMN, endotoxin thiols, Mg.sup.++ and cytokines III NADPH Ca.sup.++, 135 kDa** endothelial (particulate) BH.sub.4 calmodulin cells __________________________________________________________________________ *Molecular weight determination by sodium dodecyl sulfate/polyacrylamide gel elecrophoresis **kilo Daltons
Isoform I has been purified and characterized by Bredt and Snyder (Proc. Natl. Acad. Sci. USA, 1989, 87: 682-685) and by Schmidt et al. (Proc. Natl. Acad. Sci. USA, 1989, 88: 365-369). Isoform II has been purified and characterized by Kawai et al. (J. Biological Chemistry, 1991, 266: 12544-47). Isoform III has been purified and characterized by Pollock et al. (Proc. Natl. Acad. Sci. USA, 1991, 88: 10480-4). Isoform-specific agents may offer the advantage of selectivity, i.e., desired therapeutic effect with fewer or more tolerable side-effects.
Compounds which act directly to regulate NO synthesis or in an indirect fashion to regulate the production of cGMP through regulation of the effect of endogenous EDRF (NO) on soluble guanylate cyclase are useful in the treatment of those disease states associated with smooth muscle and smooth muscle tone, especially those involving airway, gastrointestinal and vascular muscle, and platelet function. Examples of such conditions include hypotension, endotoxemia, shock, sepsis, rhinitis, hypertension, and cerebral vasoconstriction and vasodilation, such as migraine and non-migraine headache, ischemia, thrombosis, and platelet aggregation, including preservation and processing of platelets for transfusions and perfusion technologies. Additional examples include atherosclerosis, diseases of the bronchial passages, such as asthma, diseases of the optic musculature, diseases of the gastrointestinal system, such as reflux esophagitis (GERD), spasm, diarrhea, irritable bowel syndrome, and other gastrointestinal motile dysfunctions. Such compounds may also find use in angioplasty and the treatment of sickle cell anemia.
Examples of known compounds that act to regulate the production of cGMP by this method may be grouped into four categories: (1) those compounds, for example, methylene blue, which directly or indirectly (through superoxide anion) oxidize EDRF (NO) and thereby inactivate it (R. J. Gryglewski et al., Nature, 1988, 320:454-6 and S. Moncada et al., Proc. Natl. Acad. Sci. USA, 1986, 83: 9164-68); (2) those agents, for example, hemoglobin, which directly bind either EDRF (NO) itself or one of its end products; (3) those agents which remove superoxide anion (O.sub.2).sup.- and other oxidants, thereby enhancing the effect of EDRF (for example, the enzyme superoxide dismutase removes superoxide anion by converting it to molecular oxygen (O.sub.2) and hydrogen peroxide); and (4) the nitrovasodilators, such as nitroglycerin, which provide nitrogen oxide to stimulate guanylate cyclase (F. Murad, J. Clin. Invest., 1986, 78: 1-5). With the exception of the nitrovasodilators, none of these categories of compounds has provided a viable therapeutic agent for the regulation of cGMP production in disease states. The nitrovasodilators, because they provide nitrogenous oxides indiscriminately to numerous target tissues, and thus lead to such complications as tolerance (A. Mulsch et al., European J. Pharmacol., 1988, 158: 191-8), may not be the ultimate therapeutic agents of choice. More recently it has been reported that N-hydroxyarginine is a substrate for the NO synthase enzyme (Steuhr et al., J. Biol. Chem., 1991, 266: 6259).